A noise suppressor (noise suppressing system) is a system for suppressing noise superimposed on a desired audio signal, and typically estimates the power spectrum of the noise component using the input signal that was converted into frequency domain, and subtracts this estimated power spectrum from the input signal to thereby suppress the noise mixed in the desired audio signal. When the power spectrum of the noise component is continuously estimated, it is possible to deal with the suppression of irregular noise. A conventional noise suppressor is disclosed in patent document 1 (Japanese Patent Application Laid-open 204175/2002), for example.
Usually, a digital signal that has been obtained by analog-to-digital (AD) conversion of an output signal from a microphone that corrects speech waves is supplied as an input signal to a noise suppressor. Mostly, in general a high-pass filter is disposed between AD conversion and a noise suppressor in order to suppress a low-frequency component that is added during speech collection with a microphone or during AD conversion. An example of such a configuration is disclosed in patent document 2 (U.S. Pat. No. 5,659,622).
FIG. 1 shows a configuration in which a high-pass filter of patent document 2 is applied to a noise suppressor of patent document 1.
Supplied to input terminal 11 is a noisy speech signal (a signal that contains a desired speech signal and noise) as a sequence of sample values. The noisy speech signal samples are supplied to high-pass filter 17 where the low-pass component is suppressed, and then are supplied to frame divider 1. Suppression of the low-pass component is an essential process in order to maintain the linearity of the input noisy speech and to present high enough signal processing performance. Frame divider 1 divides the noisy speech signal samples into frames of a specified number of samples and transmits them to windowing processor 2. Windowing processor 2 multiplies the divided frame of noisy speech samples by a window function and transmits the result to Fourier transformer 3.
Fourier transformer 3 performs a Fourier transform on the windowed, noisy speech samples to divide the samples into a plurality of frequency components and multiplex the amplitude values and supplies them to estimated noise calculator 52, spectral gain generator 82 and multiplex multiplier 16. The phases are transmitted to invert Fourier transformer 9. Estimated noise calculator 52 estimates the noise for each of the supplied multiple frequency components and transmits them to spectral gain generator 82. As an example of noise estimation, there is a method of estimating the noise component by weighting the noisy speech based on the past signal-to-noise ratio, the detail being described in patent document 1.
Spectral gain generator 82 generates individual spectral gains for multiple frequency components, in order to produce enhanced speech with noise suppressed by multiplying the noisy speech by the coefficients. As one example of generating spectral gains, the least mean square short period spectrum amplitude method in which the mean square power of enhanced speech is minimized has been widely used. Details are described in patent document 1.
The spectral gains generated for individual frequencies are supplied to multiplex multiplier 16. Multiplex multiplier 16 multiplies the noisy speech supplied from Fourier transformer 3 and the spectral gain supplied from spectral gain generator 82 for every frequency, and transmits the products as the amplitudes of the enhanced speech to inverse Fourier transformer 9. Inverse Fourier transformer 9 performs inverse Fourier transformation making use of the enhanced speech amplitudes supplied from multiplex multiplier 16 and the phases of the noisy speech supplied from Fourier transformer 3 and supplies the result as enhanced speech signal samples to frame synthesizer 10. This frame synthesizer 10 synthesizes output speech samples of the current frame using the enhanced speech samples of the neighboring frame and outputs the result to output terminal 12.